What the Color of Corn Can Tell You

A friend of mine from New York City called the other day and said he had seen a report that the Midwest had yellowing corn. He had thought to himself, "Isn't corn supposed to be yellow?" It gave me a chuckle and I explained the report was probably referring to the leaves -- which should be green in spring and summer.

I dug up this excerpt from a June, 2007 article from Iowa State University Agronomy Extension about yellow corn foliage including a section on purple corn foliage. Spring 2007 was wet, much like this year and color differences in young corn can mean one of two things. For yellow corn, it can be either an indication of crop stress from excessive water or nitrogen deficiency. Purple corn can mean a phosphorous deficiency or stifled root development.

The remedies are pretty simple and the trick to determining if the coloration is related to excess water or to nutrient deficiency is to wait until the soil dries out. If the plants do not return to normal, healthy green coloration, you may need to apply supplemental N or P.

Most of Iowa was saturated with precipitation amounts that are from normal to two times the normal amount since mid-April. Although a good share of Iowa's corn appears green and healthy, reports of yellow and purple corn are coming from several parts of the state. Differences in overall vigor and growth between corn following corn and corn following soybean are even more obvious than in prior years. In the long run, these could contribute to the yield penalty for corn following corn. Long-term data has shown that in good years, corn following corn can yield close to or the same as corn following soybean. In stressful years though, corn following corn will not be able to compete with corn following soybean.

Yellow Corn --

Cool, wet weather is a major cause of the yellow corn seen this growing season as it is a symptom of crop stress. Yellow corn does not necessarily point to a nitrogen deficiency. Corn grown in waterlogged soil turns yellow and may die if conditions are severe. Nitrogen and potassium deficiencies, herbicide applications, and soil compaction may all contribute to leaf yellowing. Leaf yellowing in most cases though is likely associated with a poorly developed root system. See yellowed plants from a water saturated field: field photograph and close-up photograph.

Once secondary roots develop and temperatures warm, plants are expected to recover. Some lasting results of yellow or stunted corn are uneven plant heights and/or stand reduction; these will both reduce yield potential if they occur. Once the soil dries, if the plants remain yellow, then nitrogen may be limited, which will also reduce yield.

Purple Corn --

Another symptom of cool air and soil temperatures, combined with wet conditions, is a purplish tint to corn seedlings. See image. Purple leaf coloring is more pronounced in some hybrids' genetics than others. Most often though, the leaf purpling is related to stress experienced by the young seedling and/or restricted root development. Phosphorus unavailability is often mentioned as a culprit for the purple leaves. Phosphorus deficiency will result in reddish purple leaves, yet it is not likely the primary cause. A reddish purple tint on leaves can be due to anything that disrupts sugars within the plant. Cool and/or compacted soils, as well as shallow planting, can each create the opportunity for purpling to be expressed in corn leaves. If root development is restricted (due to temperature or seedbed problems), then the observed symptoms are simply an expression of this since the plant is not developing normally.

Whenever we see purple corn seedlings we can assume one of two things: either the plant is not translocating sugars well or there is a phosphorus deficiency. Although the plants are purple, phosphorus is not likely deficient in the soil. The plants are simply responding to wet conditions in which root growth was stalled for a period of time. The plants should grow out of this condition with no significant long-term impact (except as noted above with yellow corn). For additional information on purple corn and cold season stress, read "Early season cold stress".

Summary --

Root restrictions that cause purpling or yellowing are expected to be temporary and seedlings should regain a healthy green color as weather conditions improve. Overall, yield should not be affected. It is good practice to check a few plants after they 'green up' to ensure that root development resumes.

[Text originally appeared in the Integrated Crop Management extension newsletter on pages 176-177 of the IC-498 (14) - June 11, 2007 issue.]